The SUB speaker is often an aftermarket fitment to expand the sound range of the speakers. The SUB is ideally run from it's own circuit and output on the android headunit. This allows for the greatest level of fine tuning at the headunit end, bass fader or better the EQ.
“When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network and Amazon Associates Program, affiliate advertising programs designed to provide a means for sites to earn advertising fees by advertising and linking to Android-Headunits.com.”